Surface Acoustic Wave Properties Research Articles

The dispersion properties of surface acoustic wave devices on AlN/LiNbO 3 film/substrate structure

Highly c-axis oriented aluminum nitride (AlN) films were deposited on z-cut LiNbO 3 substrates by reactive rf magnetron sputtering. The crystalline properties investigated by X-ray diffraction (XRD) revealed that AlN film with (0 0 2) preferred orientation was improved by an increase of the deposition time within the experimental range. However, the surface morphology of AlN film measured by scanning probe microscopy (SPM) showed that the roughness was getting worse with increase of deposition time. Surface acoustic wave (SAW) properties, measured by a network analyzer in the structure consisting of highly c-axis AlN films on z-cut LiNbO 3 substrates, were investigated. The phase velocity (V P) was significantly increased by the increase of h/ λ, where h is the thickness of AlN film and λ is the wavelength. However, the insertion loss (IL) of SAW filters was also increased by the increase of h/ λ. Experimental results on the temperature characteristics of SAW devices are also presented.

Surface acoustic wave properties of freestanding diamond films

"Ideal" diamond has the highest acoustic velocity of any material known, and is of great interest as a substrate material for high frequency surface acoustic wave (SAW) device structures. However, little is known of the acoustic wave propagation properties of polycrystalline diamond grown by chemical vapour deposition (CVD) techniques, the commercially accessible form of this material. We report on propagation of laser-generated SAW on three forms of freestanding CVD diamond samples, "white" polycrystalline, "black" polycrystalline, and "highly oriented" diamond. Despite differing sample nature, SAW waves propagating along the smooth (nucleation) side of the diamond showed similar velocities in the range 10600-11900 ms(-1). These results are discussed in terms of the potential of each form of CVD diamond for SAW device fabrication.

Laser-generated nonlinear surface wave pulses in silicon crystals

The absorption-layer method for inducing pressure shocks is employed to generate finite-amplitude, broadband surface wave pulses in crystalline silicon. Spectral evolution equations are used to compute the wave form distortion from the first to the second measurement location, and the results are shown to be in quantitative agreement with the measured data. The measurements also confirm that a nonlinearity matrix which describes the coupling of harmonics provides a useful tool for characterizing wave form distortion. In the (001) plane, the measurements show that the longitudinal velocity wave forms develop rarefaction shocks along $[100]$ and compression shocks along $26\ifmmode^\circ\else\textdegree\fi{}$ from $[100].$ In the (110) plane, compression shocks are observed in the longitudinal velocity wave forms in the direction $37\ifmmode^\circ\else\textdegree\fi{}$ from $[100],$ whereas rarefaction shocks are seen along $[11\ifmmode\bar\else\textasciimacron\fi{}0].$ The results in the (001) and (110) planes are consistent with sign changes in the nonlinearity matrix elements. In the (111) plane, the measured wave form distortion is consistent with the phase changes associated with the computed complex-valued matrix elements. In particular, the characteristics of propagation in the $[112\ifmmode\bar\else\textasciimacron\fi{}]$ and $[1\ifmmode\bar\else\textasciimacron\fi{}1\ifmmode\bar\else\textasciimacron\fi{}2]$ directions are shown to differ. This specific case is proved to follow from a more general result based on the symmetry properties of surface acoustic waves in this plane. In all the planes, it is demonstrated that, unlike bulk waves, the peak acoustic amplitude of surface waves can increase as they propagate, thereby allowing large stresses to be generated at surfaces. Finally, the power flux and total power of the pulses are shown to be substantially higher than in previous reports.

A new triply rotated quartz cut for the fabrication of low loss IF SAW filters.

Analysis of the quartz properties shows the existence of unexplored angular domains for which Rayleigh waves can be efficiently excited, exhibiting physical characteristics better than the ones of the (ST,X) cut. This paper presents a family of quartz cuts allowing significant improvements of surface acoustic wave (SAW) devices on quartz. A first set of experiments has been performed, confirming the theoretical predictions of the basic properties of SAW on these cuts. A second set of measurements then was achieved to refine the identification of coefficients needed to perform industrial SAW design. A demonstration of the improvements accessible using this new cut is presented. A low loss SAW filter working at 71 MHz has been fabricated using smaller aluminum thickness than that for standard quartz cuts, and exhibiting all the properties required for its industrial implementation.

A novel copper damascene technique for power loaded SAW structures

ABSTRACTThe damage of finger electrodes of surface acoustic wave (SAW) structures due to material transport (acoustomigration) forming voids and hillocks is strongly associated with the SAW stress field, the temperature and properties of the fingers material and their configuration. By application of Cu thin films a significant higher performance of SAW structures with respect to power durability, reliability and lifetime is obtained. In the present paper Cu finger electrodes of a special power SAW test structure were fabricated in trenches of STX quartz substrates using the copper damascene technique. In comparison with conventional finger electrodes located on the substrate surface, such an embedded structure enables some new features regarding their acoustical and acoustomigration behavior. So high power SAW load cannot cause fatal failures by shorts between adjacent fingers. This fact is specially important for SAW devices in the GHz range. The trench structuring into the substrate was carried out by reactive ion (RIE) or ion beam etching (IBE) technique using a metallic hard mask. Etched trenches were filled with a conductive Ta-Si-N / Cu-layer system by magnetron sputtering in a cluster tool, and structured by a chemical-mechanical polishing (CMP) process. Subsequently, after cleaning an insulating Ta-based barrier (system Ta-Si-O / Ta-Si-N) layer was deposited on the wafer surface. This covering layer also acts as a protective coating. Such a metallization system enables sufficient bonding properties using Al-wires. First results of electrical measurements show that travelling SAW could be excited in quartz substrates.

Freestanding CVD diamond elaborated by pulsed-microwave-plasma for ZnO/diamond SAW devices

In previous work, the feasibility of developing high performance AlN/diamond surface acoustic wave (SAW) devices has been demonstrated using the unpolished nucleation side of freestanding CVD diamond. This process shows the advantage of avoiding the diamond polishing, which is a tedious and a time-consuming technological step. As alternative, we propose the use of a diamond substrate obtained by two stages of growth in pulsed MPACVD to constitute two superposed layers. The first one was deposited at optimal conditions with 2% CH 4 in H 2 gas mixture, leading to a high diamond quality. The second diamond layer is used as a support in order to get a freestanding diamond. It has a lower quality because it's obtained by increasing the CH 4 content, thus the growth rate. The first diamond layer combined with piezoelectric film defines the SAW properties. Its thickness is chosen as a function of the acoustic wave penetration depth in order to limit their propagation only in the first diamond layer. Aluminium inter-digital transducers (IDTs) were deposited by conventional contact UV photolithography on the nucleation side of the freestanding diamond. Piezoelectric ZnO film was then deposited by reactive magnetron sputtering to obtain the ZnO/IDT /diamond structure. Electrical characterization of the SAW device exhibits high filtering properties. The following parameters are obtained with 32 μm wavelength, 3 μm ZnO and 25 μm diamond first layer thickness: acoustic phase velocity: v ϕ=9696 m/s, electromechanical coupling coefficient: K 2=0.75%, temperature coefficient of frequency: TCF=29 ppm/°C.

The influence of Cd doping on the surface acoustic wave properties of Sm-modified PbTiO 3 ceramics

The Sm-modified lead titanate ceramics with a composition of (Pb 0.85− x Cd x Sm 0.1)(Ti 0.98Mn 0.02)O 3; x=0.01–0.08 were prepared by conventional mixed-oxide method. Dielectric and piezoelectric properties of these doped ceramics were measured at room temperature. Microstructural and phase content analyses have been carried out using SEM and XRD. The Curie point versus the amount of Cd composition was also investigated. Surface acoustic wave (SAW) properties, including phase velocity, electromechanical coupling coefficient and temperature coefficient of frequency, were measured. The experiments successfully showed that Cd additive is helpful to obtain higher phase velocity and high electromechanical coupling coefficient, and improve the temperature coefficient of frequency (TCF). In addition, it also reduces the sintering temperature of processing. The SAW properties of our samples ( V p , IL, k 2, and TCF) compare well to the commercially-made PZT and PT samples.

Strontium doping effects on the characteristics of Sm-modified PbTiO3 ceramics

Abstract In this paper, Sr dopants are introduced in the Sm-modified lead titanate (PbTiO 3 ) ceramic systems with a composition of (Pb 0.88− x Sr x Sm 0.08 )(Ti 0.98 Mn 0.02 )O 3 ; x =0.05–0.25 using conventional mixed-oxide method. Dielectric and piezoelectric properties of these doped ceramics were measured at room temperature. Microstructural and compositional analyses have been carried out using scanning electron microscope (SEM) and X-ray diffraction (XRD). The Curie point versus the amount of Sr composition was also investigated. Surface acoustic wave (SAW) properties, including phase velocity and electromechanical coupling coefficient, were measured. The experiments successfully showed that Sr additive is helpful to obtain higher phase velocity and high electromechanical coupling coefficient. The SAW properties of our samples ( V p , k 2 ) are better than the commercially-made PZT and PT samples.

Structural, morphological and acoustic properties of AlN thick films sputtered on Si(001) and Si(111) substrates at low temperature

Polycrystalline AlN thick films were deposited on Si(001) and Si(111) substrates by reactive radio frequency sputtering technique at low temperature. The structure and the morphology of the films were investigated by X-ray diffraction, scanning electron microscopy and atomic force microscopy techniques. These measurements showed that the AlN films were highly c-axis oriented, with low surface roughness. The surface acoustic wave (SAW) properties of the films were investigated: a mean value of 3.8×10 −12 C/N was estimated for the piezoelectric strain constant d 33; the phase velocities of SAWs propagating in polycrystalline AlN/(001)[110]Si and AlN/(111)[1–10]Si structures, for different film thicknesses, were calculated and found to be in good agreement with the theoretical velocities evaluated for SAWs propagating in single crystal AlN/Si structures.

Influence of spinel substrate and over-layer for enhanced SAW and AO properties with KNbO3 thin film.

Surface acoustic wave (SAW) propagation characteristics have been studied using modeling calculations for a potassium niobate (KNbO3) thin film-layered structure with (001) and (110) orientation on a single crystal spinel (MgAl2O4) substrate, and a spinel buffer layer on silicon. Variation in the electromechanical coupling and acoustic attenuation has been compared. A significantly high value of coupling factor (k2max = 23%) is obtained for the (001)KNbO3/spinel structure by introducing an optimum thickness of spinel over-layer for potential wide bandwidth SAW device applications. The dispersion characteristics with the (110) KNbO3 orientation indicate an initial peak in the coupling coefficient value (k2max = 8.8%) at a relatively low KNbO3 film thickness that appears attractive for fabricating devices with thinner films. The KNbO3 film with (001) orientation is found attractive for efficient acousto-optic (AO) device application with the formation of a symmetric waveguide structure (spinel(0.5 microm)/KNbO3(1.0 microm)/spinel). A high value of k2 = 23.5% with 50% diffraction efficiency has been obtained for the spinel(0.5 microm)/KNbO3(1.0 microm)/spinel structure at 1 GHz SAW frequency and 633 nm optical wavelength at a very low input drive power of 15.4 mW.

Mg/sub x/Zn/sub 1-x/O: a new piezoelectric material

Piezoelectric ZnO thin films have been successfully used for multilayer surface acoustic wave (SAW) and bulk acoustic wave (BAW) devices. Magnesium zinc oxide (Mg/sub x/Zn/sub 1-x/O) is a new piezoelectric material, which is formed by alloying ZnO and MgO. Mg/sub x/Zn/sub 1-x/O allows for flexibility in thin film SAW device design, as its piezoelectric properties can be tailored by controlling the Mg composition, as well as by using Mg/sub x/Zn/sub 1-x/O/ZnO multilayer structures. We report the metal-organic chemical vapor deposition (MOCVD) growth, structural characterization and SAW evaluation of piezoelectric Mg/sub x/Zn/sub 1-x/O (x<0.35) thin films grown on (011~2) r-plane sapphire substrates. The primary axis of symmetry, the c-axis, lies on the Mg/sub x/Zn/sub 1-x/O growth plane, resulting in the in-plane anisotropy of piezoelectric properties. SAW test devices for Rayleigh and Love wave modes, propagating parallel and perpendicular to the c-axis, were designed and fabricated. Their SAW properties, including velocity dispersion and piezoelectric coupling, were characterized. It has been found that the acoustic velocity increases, whereas the piezoelectric coupling decreases with increasing Mg composition in piezoelectric Mg/sub x/Zn/sub 1-x/O films.

Evaluation on Micro Cracks in Ceramic Bearing Balls by Using the Floating Resonance of Surface Acoustic Waves

Although resonant ultrasound spectroscopy is useful for testing the surface and the inside of objects, the acoustic properties (resonance frequency, mode amplitude ratio, attenuation, etc.) are disturbed by the contact made with supports and transducers. To eliminate this disturbance, we developed the floating resonance (FR) method in which the acoustic properties of bulk and surface acoustic waves (SAWs) are evaluated using laser ultrasound after floating the objects, thus avoiding the contact with the supports and transducers. In this work we applied the FR method to detect artificial flaws on the surface of ceramic bearing balls and a slit as shallow as 50 µm was successfully detected from the attenuation of SAWs after multiple round trips with as many as 20 turns.

Investigations on temperature controlled monolithic integrated surface acoustic wave (SAW) gas sensors

The paper presents examinations on temperature behavior of mass sensitive surface acoustic wave (SAW) gas sensors. The investigations were realized on dual-delay-line SAW-devices based on AlN as piezoelectric layer. In order to measure temperature, a thin film resistor was placed on the surface of the SAW-device. To heat up the sensor device a monolithic integrated polysilicon heating resistor was used. As mass sensitive layers different polymers were deposited using a spin-on-process. For the detection of aromatic compounds like benzene, toluene and xylene suitable imprinted polymers were used. To examine sensitivity and selectivity of alcoholic compounds a polyurethane was deposited as mass sensitive layer. In our investigations, the effect of gas adsorption on the SAW transmission behavior was studied. Different atmospheres caused varying damping behavior and thus energy losses of the transmission line influenced the surface temperature of the mass sensitive layer. Growing demands on quality standards demand improvements in sensitivity, selectivity and stability of gas sensors. Because of the temperature influence on physical properties of surface acoustic waves, the temperature behavior of the SAW-devices was examined. Further on, the effects of cross sensitivity were studied according to different temperature behavior of adsorption. As a result, a higher sensitivity and stability could be achieved by controlling and changing temperature during the measurement in comparison to uncontrolled devices.

Properties of pseudo surface acoustic wave on (-74/spl deg/)YX cut quartz

Pseudo surface acoustic wave properties on (-74/spl deg/)YX cut quartz have been calculated and measured. It was found that this cut belongs to the minimal diffraction cuts, and the reflection coefficient of a single aluminium electrode is about four times larger compared to that of the ST cut. Temperature compensation exists if, for a given aluminium layer thickness, a proper free surface length is used. This cut is free of spurious transverse modes.

Properties of pseudo surface acoustic wave on (−74°)YX cut quartz

Pseudo surface acoustic wave properties on (−74°)YX cut quartz have been calculated and measured. It was found that this cut belongs to the minimal diffraction cuts, and the reflection coefficient of a single aluminium electrode is about four times larger compared to that of the ST cut. Temperature compensation exists if, for a given aluminium layer thickness, a proper free surface length is used. This cut is free of spurious transverse modes.

Effects of strontium on the surface acoustic wave properties of Sm-modified PbTiO 3 ceramics

The Sm-modified lead titanate ceramics with a composition of (Pb 0.88− x Sr x Sm 0.08)(Ti 0.98Mn 0.02)O 3; x=0.05–0.25 were prepared by conventional mixed-oxide method. Surface acoustic wave (SAW) properties, including phase velocity, electromechanical coupling coefficient and temperature coefficient of frequency, were measured. The experiments successfully showed that Sr additive is helpful to obtain higher phase velocity and high electromechanical coupling coefficient. The SAW properties of our samples ( V p , k 2) are better than some commercially-made PZT and PT samples.

Some properties of surface acoustic waves in anisotropic-coated solids, studied by the impedance method

The impedance method provides a convenient framework for analysing the velocity spectrum of surface acoustic waves, propagating in anisotropic-coated substrates within the subsonic range where there is no energy leakage into the substrate. The dispersion equation determines subsonic wave velocities as zeros of eigenvalues of the aggregate impedance which are real monotonic functions of velocity at any fixed wavenumber. The impedance method lends insight into the properties of a coated substrate spectrum by elucidating its connection to the individual acoustic parameters of the substrate and the coating layer materials. By using this approach, certain bounds and basic trends for the dispersion of velocity branches in the coated substrates are revealed. The low-frequency and high-frequency dispersion is explored in detail. The criterion is examined for either speeding or slowing trend of the low-frequency linear dispersion of the velocity branch evolving from the Rayleigh velocity of the substrate. Specific behaviour of this branch due to the higher-order dispersion in substrates coated by a relatively light or dense, “slow” or “fast” layer is interpreted. The impedance method facilitates a similar analysis for various layered structures, for instance, substrate with multilayered coating, a plate coated on both sides, and a layer embedded between two substrates.

Informational properties of surface acoustic waves generated by laser-material interactions during laser precisionmachining

Laser precision machining is primarily a computernumerical control based technology; therefore, the need foron-line process monitoring and control system is very high,mainly because, under usual circumstances, the operator has tomake a host of complex decisions, based on a trial-and-errormethod, to set the process control parameters related to the laser,optics, workpiece material and motion system. However, thecrucial element of any control system is information aboutthe process and system dynamics. The problem of choosingreliable and, at the same time, physically observed information onprocess parameters becomes more complicated in the case of laserprecision machining where the laser-material interactionsgenerate a number of emissions with a different physical nature.To capture the informational properties from the measuredsignals and parameters for the purpose of monitoring and controllingthe process, a thorough understanding of the entirelaser-machining system performance and the laser-materialinteractions is required. This paper describes a method for theanalysis of informational properties of surface acoustic wavesto generate the knowledge as an informational basis for futuredevelopment of on-line monitoring and process control systems.The study involves measurement of the acoustic emission signalfrom the laser-material interaction zone, the statistical andspectral signatures and the pattern recognition analysis to select informational parameters, which could reliably correlate withvariations within the incident laser pulse energy.

Synthesis and surface acoustic wave properties of AlN films deposited on LiNbO3 substrates.

The c-axis-oriented aluminum nitride (AIN) films were deposited on z-cut lithium niobate (LiNbO3) substrates by reactive RF magnetron sputtering. The crystalline orientation of the AIN film determined by x-ray diffraction (XRD) was found to be dependent on the deposition conditions such as substrate temperature, N2 concentration, and sputtering pressure. Highly c-axis-oriented AIN films to fabricate the AIN/LiNbO3-based surface acoustic wave (SAW) devices were obtained under a sputtering pressure of 3.5 mTorr, N2 concentration of 60%, RF power of 165 W, and substrate temperature of 400 degrees C. A dense pebble-like surface texture of c-axis-oriented AIN film was obtained by scanning electron microscopy (SEM). The phase velocity and the electromechanical coupling coefficient (K2) of SAW were measured to be about 4200 m/s and 1.5%, respectively. The temperature coefficient of frequency (TCF) of SAW was calculated to be about -66 ppm/ degrees C.

Properties of Surface Acoustic Waves in AlN And GaN

ABSTRACTThe surface acoustic wave velocities, electromechanical coupling coefficients, and the spatial distributions of both elastic displacement and electric potential have been calculated for various configurations of gallium nitride and aluminum nitride. The electromechanical coupling coefficient values of 0.13 % in GaN and 0.29 % in AlN have been predicted. The maximum electromechanical coupling coefficient values of 0.24 % at Euler angles (0, 54°, 90°) in GaN and 1.08 % at (0, 53°, 90°) in AlN have been found. For GaN layer-on- sapphire substrate structures, the SAW velocity and electromechanical coupling coefficient have been calculated as functions of layer thickness and acoustic wavelength. The experimentally measured values of the surface acoustic wave velocity and electromechanical coupling coefficient are in satisfactory agreement with the calculation results.